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研究生(外文):Jian-Hong Lin
論文名稱(外文):Improving effects of poly (vinyl pyrrolidone) addition on Pt/copper phosphate hydroxide/carbon black bifunctional catalysts for fuel cells
外文關鍵詞:Copper phosphate hydroxidePt catalystElectric resistanceCarbon blackDirect methanol fuel cells (DMFCs)
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本實驗首先將PVP與碳黑混合改善其分散性,再把碳黑置於雙氧水中使其表面富含OH-官能基,之後將氫氧磷酸銅沉積於改質的碳黑上增加其導電性,再把白金還原於其上形成Pt/CuPH-PVP-C觸媒,最後再將此觸媒做熱處理100℃ 3小時。再與未添加PVP之觸媒相互比較,並利用X光繞射(XRD)、場發射掃描式電子顯微鏡(FE-SEM)、場發射穿透式電子顯微鏡即時富利葉轉換(FE-TEM Live-FFT)、X-ray光電子能譜(XPS)、感應耦合電漿質譜儀(ICP-MS)、富利葉轉換紅外線光譜儀(FTIR)與循環伏安法(CV)進行材料分析。
  在電阻方面,Pt/CuPH-PVP-C 3個觸媒明顯低於Pt/CuPH/CB觸媒,FTIR部分也顯示氫氧磷酸銅成功地批覆在改質的碳黑上,透過FE-TEM和FE-SEM可觀察到Pt/CuPH-PVP-C分散性有提升,透過TEM也可以觀察到白金顆粒大小為2.62~3.52奈米。在CV測試中,相較於未添加PVP者氫氣吸/脫附反應電化學活性表面積在添加適量之觸媒表現,由752 提升至895(cm2/mg),在額外添加了兩倍量的碳黑於觸媒,電化學活性表面積更由1067提高至1290(cm2/mg);在甲醇氧化反應中,添加PVP之觸媒最高的質量活性由262提升至328(A/gPt)和最低的起始電位由0.315降至0.256(V)且沒有明顯反應峰出現,顯示無一氧化碳毒化現象,由於這些觸媒中的Cu2+能產生Cu-OH,能進一步把Pt上的CO氧化成CO2故達到解毒效果;在1000圈循環壽命測試下,觸媒之殘留電化學活性表面積仍達80.1%,然而添加過多的PVP會使氫氧磷酸銅較難沉積於碳黑上使白金分散又變差終而降低觸媒效果。此外,在額外加入碳黑後可使阻抗下降進而增加觸媒的催化活性。
在MEA測試下,與Pt/CuPH/CB, Pt/C比較,Pt/CuPH-PVP-C表現出較高的功率密度且有較高之開路電位,除了因具有較低之阻抗外,這些觸媒裡的Cu2+在低電位活化水而產生Cu-OHads,因而創造機會使在Pt上的CO反應成CO2和H+,這些論點也適用於解釋MOR的種種現象,顯然,在DMEC裡的觸媒表現,電阻與CO中毒現象扮演了主要的角色,由添加PVP改善調製的Pt/CuPH-PVP-C確實提高了導電率和解毒功效,而已呈現可接受之性能。
Direct methanol fuel cells (DMFCs) are widely used in small-scale portable applications, and Pt is the superior and major catalyst in them, due to its excellent catalytic activity and chemical stability. However, Pt catalysts have two major problems should be overcome in DMFCs. One is that CO poisoning effects on the Pt catalysts should be deleted, and the other is that the amount of Pt should be reduced by increasing its activity, in order to reduce the cost of devices. Based on the previous study in our laboratory the copper phosphate hydroxide (Cu2PO4(OH)), named libethenite and assigned to CuPH, with the excellent performance in oxidation of olefins and alcohols, reveals hydroxyl radicals which could be used as a catalyst to alleviate CO poisoning effects.
Phosphate salts have shown poor electronic conductivity. In order to enhance its conductivity, the precipitation of CuPH was directly deposited on the surface modified carbon black (CB) to form CuPH/CB. Then, nano-sized Pt was reduced on CuPH/CB to form Pt/CuPH/CB catalyst and finally annealed at 100℃ for 3 hours. However, the agglomeration of catalyst was seriously found. Therefore, in this study, we try to add poly(vinyl pyrrolidone)(PVP) for dispersing catalysts and hence enhancing the detoxifying function as well as electrochemical performance. The features of these catalysts were characterized by the electric resistance measurement, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), field emission transmission electron microscopy – live fast Fourier transform (FETEM-Live FFT), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), Fourier transform infrared (FTIR) spectroscopy and cyclic voltammetry (CV).
It is found that the resistance is reduced from 95.2 to 79 Ω, the ECSA is enhanced from 752 to 805.0 cm2/mg, the mass activity is improved from 262.12 to328 (A/gPt), and the on-set potential is lowered from 0.315 to 0.256V (vs. Ag/AgCl), in the methanol oxidation reaction. Besides, no forward or reverse current peaks are observed, indicating no CO poison effects. For 1000 cyclic life tests, the retained capacity of Pt/CuPH-PVP-C is reaching up to 80.1%.
In membrane electrode assembly (MEA) tests, all prepared Pt/CPH-PVP-C catalysts reveal the greater power density and open circuit voltage than Pt/CuPH/CB and Pt/C (Alfa), since Cu2+ in these catalysts activates the water at low potential to yield Cu-OHads and detoxify the nearby nano-sized Pt poisoned by CO to create the opportunity for forming CO2 and H+, beside the enhanced conductivity. This argument also reasons results of MOR tests out. Obviously, both electric resistance and CO poisoning effect play the major role on the performance of catalysts in DMFC, and the Pt/CuPH-PVP-C improved from Pt/CuPH/CB has revealed the acceptable performance due to its well-dispersed catalysts with enhanced conductivity and detoxifying function.
摘要 i
Abstract iii
Table captions vii
Figure captions viii
Chapter 1 Introduction 1
Chapter 2 Materials and methods 6
2.1 Materials 6
2.2 Preparation of CuPH-PVP-C and Pt/CuPH-PVP-C 7
2.2.1 CuPH-PVP-C 7
2.2.2 Pt/CuPH-PVP-C 7
2.3 Materials characterization 9
2.3.1 Electric resistance measurement 9
2.3.2 FTIR 10
2.3.3 The crystallinity, morphology and microstructure 10
2.3.4 ICP-MS 10
2.3.5 XPS 10
2.4 Electrode preparation and characterization 12
2.4.1 Electrode preparation 12
2.4.2 Electrochemical measurement 12
2.5 Preparation of membrane electrode assembly (MEA) for DMFC single cell polarization test 14
Chapter 3. Results and discussion 15
3.1 Characterization of the CuPH-PVP-C supports and Pt/ CuPH-PVP-C catalysts 15
3.1.1 Resistance measurement 15
3.1.2 FTIR (fourier transform infrared) analysis 16
3.1.3 XRD (X-ray diffraction) 18
3.1.4 FE-SEM (field-emission scanning electron microscope) 21
3.1.5 FE-TEM(field-emission transmission electron microscope) 25
3.1.6 ICP-MS (inductively-coupled plasma mass spectrometry) 31
3.1.7 Chemical states of platinum and copper 33
3.2 Electrochemical characterization 37
3.2.1 HOR (hydrogen oxidation reaction) 37
3.2.2 MOR (methanol oxidation reaction) 42
3.2.3 CO-stripping and Chronoamperometry 46
3.2.4 Membrane electrode assembly (MEA) 47
Chapter 4 Summary and conclusions 50
References 52
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